There are already known digital DC voltage meter devices for the measurement of voltages on a locomotive, all of which are hand held models. However, these devices have several important disadvantages.
One such disadvantage of previous meter models is their dependence on an external power supply in order operate the meter and obtain readings. This can be highly impracticable because in the field most locomotives operate accessories on 74 volts DC, which is not a common voltage range. Therefore, it would be highly useful to locomotive applications for the meter to be able to utilize the existing 74 volts DC power supply.
A second disadvantage of the hand held portable models relates to safety concerns. Portable units designed for measuring these voltages require the user to select voltage ranges of millivolts, for shunt volt readings (0-75 millivolt potential) and volts .times.100, for the main generator voltage readings (0-1500 volt potential). Improper selection of range is very dangerous as an overload would not only destroy the meter but could also cause considerable injury to the person taking the reading.
A third disadvantage to previous digital, DC voltage meters is their inability to operate in extreme temperatures. Field temperatures can range from 20 degrees Fahrenheit below zero to 120 degrees Fahrenheit above zero.
Because these readings give a quick look at the operating performance and the possibility of potential failure of the generator, it is important that the information be readily available anytime an evaluation or running checkup is required. A permanent installation on the power plant, where the test jacks are normally located, assures immediate access to this information.